Complete background information is set forth in my U.S. Pat. No. 4,162,707. Generally speaking, in the typical in situ leaching operation, wells are drilled into a mineral or metal value-bearing formation, in particular one bearing uranium, and a lixiviant is flowed between the wells to dissolve the desired values into the lixiviant. The pregnant lixiviant is produced to the surface, where it is thereafter treated to recover the desired values from the lixiviant. This treatment is usually accomplished by ion exchange. The barren solution, which is the leachate solution after ion exchange, can be recycled into the leaching cycle by adding water and/or lixiviant reagents to restore the barren solution to full utility. As a matter of practice, at least 2% of the lixiviant stream must be withdrawn during each cycle so that ground water will be drawn into the formation. Usually about 5 to 10% of the stream must be withdrawn for each cycle to avoid migration of ammonium ions, etc., outside of the formation being leached.
Unfortunately, many of the known and highly effective lixiviants not only leach the desired values from the formation, but also react with certain formations to give up chemical substances which remain in the formation after the lixiviants pass therethrough. Where ammonium carbonate or ammonium bicarbonate is used in the leach solution, the formation will be contaminated with ammonia, producing the potential of water pollution through contamination of aquifers flowing in or near the formation. Prior to my invention as disclosed in U.S. Pat. No. 4,162,707, none of the processes proposed for rapid restoration of ammonia-contaminated formations seemed to be workable. The method I discovered, which is disclosed in my U.S. Pat. No. 4,162,707, involves the treatment of the formation with a halogenated restoration fluid which preferably comprises chlorinated water and/or a hypochlorite solution to restore the formation quickly and completely to an ecologically acceptable level of ammonium ion concentration. The restoration fluid can also be used to make up fresh restoration fluid for recycle without an additional treatment operation. This is because the chlorine or hypochlorite added to the restoration fluid that is withdrawn through the production well after it has passed through the formation and prior to its being recycled to the injection wells to be introduced to the formation once again attacks the ammonia in the restoration fluid drawn up through the production wells and eliminates it from the solution prior to recycling. I also disclose that the rate efficiency of this restoration process may be improved by adjusting the pH of the chlorinated water and/or hypochlorite restoration fluid with a base such as calcium hydroxide or sodium hydroxide to a value of 7 to 13.